Gamma ray laser

ABSTRACT

Gamma ray laser device, wherein a coherent beam of gamma radiation is generated through stimulation of gamma transitions in excited atomic nuclei in a biologically shielded and low temperature closed resonating cavity.

United States Patent 1111 3,561,933

[72] Inventor Lawrence]. Piekenbrock [56] m t IN gggfg UNITED STATESPATENTS gig;- f 1968 3,233,107 2/1966 Senett 250/84 [45] Patented Mu21971 3,234,099 2/1966 Baldwin m1. 250 34 73] Am m 1 3,281,600 10/1966Vali m1. 250/84 g Boa; c'olo 3,430,046 2/1969 Eerkens 250/84 PrimaryExaminer-Archie R. Borchelt Attorneys-Albert C. Johnston, Robert E.lsner, Lewis H.

Eslinger and Alvin Sinderbrand [$4] GAMMA RAY LASER 5 Claims, 1 DrawingFig.

[52] US. Cl. 250/84,

331/945 ABSTRACT: Gamma ray laser devlce, wherem a coherent [51] Int. CLG2lh 3/00 beam of gamma radiation is generated through stimulation of[50] Fleldofseerch 250/84; gamma transitions in excited atomic nuclei ina biologically 331/845 shielded and low temperature closed resonatingcavity.

This invention relates to methods and devices for the production ofcoherent beams of ultrahigh frequency electromagnetic radiation andparticularly to methods and devices for generating a coherent beam ofgamma radiation of energy greater than 1 kilo electron volt.

To date, the field of operation of lasers and masers has been limited tothe generation of light of wavelengths below the near ultraviolet rangeor of even longer wavelength light, heat and microwaves. In suchdevices, operability is dependent upon the stimulated emission ofradiation from the electron shells surrounding the atom and suchelectron shell emission poses an inherent obstacle to the production ofcoherent ultrahigh frequency radiation of the type herein of concern.

This invention, in its broad aspects, employs the utilization ofstimulated emission from the nucleus of the atom to effect thegeneration of a coherent beam of gamma rays which have wavelengths thatare several orders of magnitude shorter than the wavelengths currentlygenerated by conventional laser devices. Such gamma radiation inherentlypossesses an energy content that is several orders of magnitude greaterthan the energy content of radiation generated by conventional lasers.

In its more narrow aspects, the subject invention includes method andapparatus for stimulating gamma transitions in excited atomic nuclei toproduce gamma radiation in a biologically shielded and low temperatureclosed resonating cavity through matching the emission process gammatransition energies to the absorption process gamma transition energies.

Among the advantages attendant, the practice of the subject invention isa permitted widespread adaptation of coherent gamma radiation toinstrumentation oriented applications to provide extremely accuratemeasurements of distance, temperature, velocity, etc.; the utilizationthereof in conjunction with existing holographic microscopy methods toeffect magnifications that far exceed anything presently available inelectron microscopes, as, for example, magnifications in the order of1,000,000,000; and the permitted use, because of theinherent extremelyrapid release of energy, as a trigger for numerous other types ofdevices. Still other advantages are permitted uses in varied fieldswhich require or could utilize extremely rapid rates of release oftremendous quantities of energy, either in discrete bursts or atpredetermined, periodically spaced intervals.

The principle object of this invention is the provision of novel andimproved methods and apparatus for effecting the generation of coherentbeams of gamma rays of energy of greater than 1 kilo electron volt.Other objects and advantages of the subject invention will-becomeapparent from the following portions of this specification and from theappended drawings which illustrate, in accordance with the dictates ofthe patent statutes, the essentials of presently preferred apparatusincorporating the principles of the invention and in which the soleFIGURE is a schematic representation of essential components ofapparatus for generating a coherent beam of gamma radiation of thecharacter described above.

As schematically illustrated in the sole FIGURE, there is provided asource of gamma radiation 10, in which gamma transitions in excitedatomic nuclei produce gamma radiation, a plurality of reflecting mirrormembers, such as the Bragg reflection mirrors 14a to 14d, to define aclosed circuit resonator cavity within which the emitted gamma rays arecontained to reenter the source and to thereby stimulate additionalgamma transitions in excited atomic nuclei and means for subjecting theatomic nuclei to selective magnetic and/or high frequencyelectromagnetic fields for matching the emission process gammatransition energies to the absorption process gamma transition energies.All of the above-named components are enclosed within a cryostat, asgenerally designated by the dotted line 12, which in turn is surroundedby appropriate biological shielding means, generally designated by thedotted line l6.

The following will serve to outline the theoretical basis for theoperation involved and to define the parameters that affect the same.

In order to sustain oscillation in a laser, it is necessary that thefollowing equation be satisfied:

Gil

AN =N2%N1 is the inverted population N N population of lower and upperlevels In radioactive decay, we can represent the population of theupper level by where:

A initial activity in disintegration/sec. r half'life of the secondlevel (3) A 2 l/r If we assume 7 r, then: N N and ANmN zAx e 2 -t/mfurthermore:

where:

T =7: A r) y statistical factor T .6937i /-r 0 wavelength of relevantradiation E =hv hv =difference in energy of incident radiation and theenergy of the excitation From equation 1 we obtain:

proximately represented-by:

where: x 6 /1 6,, Debye Temperature Er= The recoil energy T= Thetemperature in I(.

In order to maximize this expression, the Debye temperature should be aslarge as possible and the operating temperature should be small withrespect to the Debye temperature.

The Zeeman effect produced by magnetic field gradients and/or the highfrequency electromagnetic fields generated in the component 20 in effectproduce emission and absorption energy line broadening which is used tominimize e in equation 10, and hence to maximize the realizable gain. Itcan be observed from the final equation 10, that the followingparameters are involved:

c is a constant;

A is the activity of the gamma ray source e is the base of the naturallog system;

I is the time;

T is the half life of the state in question;

E is the transition of the state in question;

6 is the difference between emitted and absorbed gamma ray energy;

b is a constant;

Iis the length of the active media;

R is the product of the reflectivities of the Bragg mirrors creating theresonant cavity;

B is the loss per centimeter.

It is readily seen that if the reflectivities of the Bragg mirrors areone, and the loss is zero, the total quantity on the left becomes large,thereby satisfying the equation. It can also be readily seen that if thehalf life 1- is small, along with the energy difference 6, this willalso help satisfy the equation. The equation also serves to define basicrequirements for source nuclei and thus permits selection of suitablematerials that have attainable limits for E, l, R and B.

In the above-described device, the gamma ray source 10 may suitablyconstitute a high purity crystal of radioactive material, or combinationthereof, having a high curie temperature so as to provide a relativelyhigh degree of recoilless emission of gamma rays. Additionally, thematerial utilized should be of such character that is not susceptible toelectron and molecular interactions with applied magnetic fields tothereby minimize, if not avoid, all induced heating. The particularmaterial employed must also have a metastable level appropriate to theparticular mode of operation and to effect the emission of gammaradiation through transitions from the metastable state to a lowerenergy level. In order to effectively achieve the latter, the crystalshould desirably be of highest obtainable purity so as to minimize, ifnot avoid, internal losses of appreciable magnitude that otherwiseinherently occur due to the presence of substantial amounts ofimpurities within. Included among suitable radioactive materials useableas the gamma ray source 10 is Sn which possesses the necessaryproperties to satisfy the above requirements and which has a halflife inthe order of 8 days. With such a material, the percentage of effectivelyrecoilless emission of gamma rays during the transition from themetastable state to a lower energy level is effectively increased byoperating at low temperatures, for example, in the order of 10 K.,within the cryostat 12.

In order to compensate for the energy differences which are normallyattendant losses occurring during or subsequent to gamma ray emission,the emission process gamma transition energies are matched to theabsorption process transition energies by subjecting the gamma raysource 10 to the action of a controlled magnetic field. This field maybe of uniform character, or may be designed to provide a predeterminedfield gradient in accordance with the exigencies ofa given installation.If a nonoscillating magnetic field is employed, internal energy levelmatching between emitting and absorbing nuclei in the active sourcematerial is effected by means of the Zeeman effect. Alternatively, suchinternal energy level matching can be effected by exposing the source 10to electromagnetic field which functions to achieve effective linebroadening and hence an increase in stimulated emission when therecoilless emission and absorption peaks are very narrow and closetogether. Either or both types of energy level matching techniques maybe utilized in any given installation.

In order to effect the necessary gain within the system, the emittedgamma radiation is reflected within a closed resonator cavitybidirectionally through the gamma ray source 10. Such desired reflectionof the gamma radiation is effected by a plurality of Bragg reflectionmirrors so arranged and positioned as to provide the requisite closedcircuit defining the resonator cavity. Although four such mirrors areschematically illustrated, a greater number may be required and suchwill be determined by the frequency of the particular gamma radiationinvolved and the particular material of which the mirrors are made. Themirrors per se are constituted by crystals in which the crystal planesare free from internal defects and may be suitably constituted by puregermanium or silicon crystals. The mirrors are mounted in a so-calledpuckered arrangement as disclosed in Resonator For An X-Ray Laser byBond, Duguay and Rentzepis, Applied Physics Letters, Vol. 10, No. 8,Apr. 15, 1967. Each of the crystal mirrors must be mounted so as topermit the adjustment of the positioning thereof, and their initialpositioning may be determined by X- ray diffraction techniques. Likewisefocusing of the beam may be effected by physical warping of the mirrorelements.

As will be apparent, one of the mirrors, for example the mirror 14a,must be so constructed so as to permit a portion of the coherent gammaray beam to escape the resonant cavity and to exit from the device.

In a unit as above described, the gamma ray source 10 is of suchcharacter as not to require the application of any external excitationin order to effect the desired degree of transition from the metastablestate to the lower energy level. That is, the source material employedis possessed of a sufficient inverted population at the metastable stateto provide, by stimulated emission, a sufficient gain to establish acoherent beam output.

As an alternative to the above, the subject device may include means 18for externally exciting the source material to provide the necessaryinverted population. One such means may comprise an auxiliary source ofneutrons or other pumping mechanism to provide sufficient additionalelectromagnetic energy as to establish the necessary inverted populationrequired to produce a stimulated gain as described above. With the useof such external excitation, a different selection and possible greatervariety of gamma ray source materials are available for use. Thecharacteristics of suitable source materials useable with externalexcitation include source materials of general character set forthabove, together with materials of much shortened half life of themetastable state as determined by the amounts of pumping energyavailable. If a neutron source is employed for external excitation, suchsource, which suitably could comprise a nuclear reactor, should beadapted to provide the necessary quantities of thermal neutrons. Whenexternal excitation is employed the device will generally operate in acyclic manner emitting bursts of energy at periodic intervals.

As now will be apparent to those skilled in this art, theabove-described method and device may be utilized in a great number ofdiverse applications. In one aspect, the device may generally be thoughtof as an energy storage system capable of very rapid release rates. Inother words, it can be viewed as a type of supercapacitor where theenergy is released in the form of coherent gamma radiation. Forinstance, calculations indicate that when a l curie source of Sn*'" isstimulated, it will release its entire energy in a period of less thannanoseconds, thereby providing an energy source of approximately 30,000megawatts, which is at least 30 times the energy release obtained fromlarge pulsed ruby lasers. Such high energy release rates could beemployed for useful purposes such as for interception and destruction ofmissiles, tunneling through mountains, well drilling, and for road andcanal building projects and the like. The coherent nature and wavelengthof the emitted radiation also lends itself to numerous instrumentationoriented utilizations such as extremely accurate measurements ofdistance,'time, velocity, and the like through adaptation ofinterference techniques already in existence for optical lasers. Inaddition, and through means of holographic methods, magnifications onthe order of 100 times that which may be achieved in electronmicroscopes are possible. Another major area of utilization for such ahigh intensity energy source would be the exciting of plasma and thetriggering of other energy releasing nuclear reactions throughcontrolled fusion or fission. In comparison with techniques presentlybeing investigated for producing controlled fusion reactions, systemsutilizing devices of the type herein disclosed would be relativelystraightforward and less expensive to build.

lclaim: 1. In the method of the steps of:

stimulating gamma transitions in excited atomic nuclei to produce gammaradiation while maintaining said nuclei at reduced temperatures tomaximize the probability of recoilless gamma ray emission; matching theemission process gamma transition energies to the absorption processgamma transition energies by subjecting said atomic nuclei to selectiveelectromagnetic fields; and

producing coherent gamma radiation,

reflecting said emitted gamma radiation within a multielement closedresonator circuit through said source.

2. The method as set forth in claim 1, including the step of externallyexciting said atomic nuclei to create a population inversion at ametastable state.

3. Apparatus for producing coherent gamma radiation comprising:

radioactive isotope crystal means characterized by a populationinversion at-a metastable state adapted to produce gamma transitions inexcited atomic nuclei;

means for exposing said crystal means to selective electromagneticfields to match emission process gamma transition energies to absorptionprocess gamma transition energies; means for selectively directingemitted gamma radiation within a multielement closed circuit resonatingcavity for stimulating additional gamma transitions to effect emissionof coherent gamma radiation; and

cryostat means enclosing said above-recited means for maintaining saidexcited atomic nuclei at reduced temperatures to maximize theprobability of recoilless gamma ray emission.

4. Apparatus as set forth in claim 3 including means for externallyexciting said crystal means to create a population inversion at ametastable state.

5. Apparatus as set forth in claim 3 wherein said external excitationmeans comprises an intense source of thermal neutrons.

gamma ray

1. In the method of producing coherent gamma radiation, the steps of:stimulating gamma transitions in excited atomic nuclei to produce gammaradiation while maintaining said nuclei at reduced temperatures tomaximize the probability of recoilless gamma ray emission; matching theemission process gamma transition energies to the absorption processgamma transition energies by subjecting said atomic nuclei to selectiveelectromagnetic fields; and reflecting said emitted gamma radiationwithin a multielement closed resonator circuit through said gamma raysource.
 2. The method as set forth in claim 1, including the step ofexternally exciting said atomic nuclei to create a population inversionat a metastable state.
 3. Apparatus for producing coherent gammaradiation comprising: radioactive isotope crystal means characterized bya population inversion at a metastable state adapted to produce gammatransitions in excited atomic nuclei; means for exposing said crystalmeans to selective electromagnetic fields to match emission processgamma transition energies to absorption process gamma transitionenergies; means for selectively directing emitted gamma radiation withina multielement closed circuit resonating cavity for stimulatingadditional gamma transitions to effect emission of coherent gammaradiation; and cryostat means enclosing said above-recited means formaintaining said excited atomic nuclei at reduced temperatures tomaximize the probability of recoilless gamma ray emission.
 4. Apparatusas set forth in claim 3 including means for externally exciting saidcrystal means to create a population inversion at a metastable state. 5.Apparatus as set forth in claim 3 wherein said external excitation meanscomprises an intense source of thermal neutrons.